This deceptively creamy low-fat dessert can be made early in the day and refrigerated for several hours before serving.
In a large saucepan, bring the milk, vanilla, tequila, cinnamon, and 2 tablespoons sugar to a boil. (If using vanilla bean, split pod and scrape the beans into the milk.) Reduce heat to a high simmer and let milk reduce to 2 cups, stirring frequently, about 20 to 25 minutes.
Place 6 four-ounce custard cups in a 2-inch-deep baking pan. Heat oven to 350 degrees.
Place 1/4 cup water and remaining 3/4 cup sugar in a heavy saucepan. Cover and bring to a boil over high heat until sugar dissolves. Uncover; reduce heat to medium high; boil until syrup darkens to a light amber color, about 10 minutes. Do not stir. Immediately pour into custard cups, swirling caramel up the sides.
Whisk together eggs in a large bowl. Slowly beat in hot reduced milk, strain mixture, then pour into custard cups.
Fill baking pan with 1/2 inch of warm water and cover with a tent of aluminum foil. Bake until custards are just set, 30 to 40 minutes. Remove pan from oven and remove custards from pan. Serve when cooled, or cover flans with plastic wrap and refrigerate until ready to unmold.
To unmold, run a knife carefully around the edge of each custard. Invert onto a plate and drizzle 1 teaspoon of coffee liqueur, if using, over each flan.
1/2 vanilla bean can be replaced with 1 teaspoon extract. 1 cinnamon stick can be replaced with 1/8 teaspoon ground cinnamon.